Nerve Conduction Studies
Core concepts
Nerve conduction studies (NCS) quantify how well a peripheral nerve conducts an impulse. The essentials are latency (time from stimulus to a defined response), amplitude (size of the response, reflecting the number of excitable fibers and their synchrony), conduction velocity (distance divided by the latency difference between two stimulation sites), and duration/dispersion. Sensory studies measure the sensory nerve action potential (SNAP); motor studies measure the compound muscle action potential (CMAP).
Patterns to recognize
A demyelinating process prolongs latency, slows conduction velocity, and may produce conduction block (focal amplitude drop across a segment) or temporal dispersion. An axonal process reduces amplitude with relatively preserved velocity and latency. Understanding this dichotomy answers most pattern-recognition questions.
Traps
- Confusing amplitude loss (axonal) with slowing (demyelinating).
- Forgetting that temperature slows conduction; cold limbs falsely prolong latencies and slow velocity.
- Mixing onset latency (sensory) with peak latency.
- Assuming a low SNAP always means nerve damage when it may reflect a technical issue or normal aging.
Rule of thumb
Velocity (m/s) = distance (mm) ÷ latency difference (ms). Example: a motor nerve stimulated at two sites 200 mm apart, with proximal latency 6.0 ms and distal latency 3.0 ms, gives a difference of 3.0 ms, so velocity = 200 ÷ 3.0 ≈ 66.7 m/s. Always subtract latencies to remove the distal segment and neuromuscular junction time.